1. Field of the Invention
The present invention relates to an electron accelerator for electrostatical acceleration of electrons emitted from a hot cathode provided in a vacuum vessel.
2. Prior Art
Electron accelerators have various industrial uses such as for use in graft polymerization, antipollution devices and material analysis. FIG. 1 shows an example of an electron accelerator. In a vacuum vessel 10, a cathode 1, a truncated-cone-shaped bias electrode 2 disposed just in front of the cathode 1, an extracting electrode 3 having a central aperture for allowing an electron beam to pass therethrough, and a group of electrostatic lenses 4 having central apertures are arranged in the preceding order along a central axis X to produced an accelerated electron beam. At an exit of the accelerates, a titanic film window 5 is formed on a wall of the vacuum vessel 10. The accelerated electron beam is detected at an electron detecting electrode 12, which is set outside the window 5.
A DC high voltage is applied to the extracting electrode 3 and the electrostatic lenses 4 through a series of a resistors 14 across which a DC high voltage power supply 16 is connected. The cathode 1 is heated by an adjustable power supply 18. A DC power supply 20 is connected between the cathode 1 and the bias electrode 2, so that the electrical potential of the bias electrode 2 is lower than that of the cathode 1. The power supply 18 and 20 receive electrical power through an insulating transformer 22 from an AC power supply 24.
When this electron accelerator is driven, the cathode 1 is made red hot and thermal electrons are extracted from the hot cathode 1 by the extracting electrode 3. Since the bias electrode 2 has the central aperture for allowing the electrons to pass and is therethrough at the above-mentioned electric potential, the bias electrode 2 focuses the electron beam through the central portion of the extracting electrode 3. The electron beam then passes through the central apertures of the electrostatic lenses 4 which not only focus but also accelerate for the electron beam. During its passage through the lenses 4, the electron beam is repeatedly converged and accelerated to form an accelerated electron beam and is emitted through the titanic film window 5 to the outside. An electric current proportional to an amount of the electron beam impinging the detecting electrode 12 is converted to an AC current by a DC/AC converter 26. This AC current is fed through an insulating transformer 28 to an AC/DC converter 30 which converts the AC current to a DC current which is then fed back to the power supply 18. This negative feedback loop controls the power supply 18 so that the cathode 1 is heated to a temperature at which the current of the accelerated electron beam remains constant.
It is noted that the detecting electrode 12 can be provided inside the vacuum vessel 10.
In such an electron accelerator, a high voltage power supply is required to obtain a high energy electron beam. For example, a DC high voltage power supply of 1 MV, 100 mA is required to obtain an electron beam of 1 MeV and 100 mA. On the other hand, since the detecting electrode 12 is kept at ground potential, the potential difference between the power supply 18 and the detecting electrode 12 is significantly large. In order to tolerate such a high potential difference, the insulating transformers 22 and 28 have to be substantially large and are, as a result, expensive. In other words, the size and cost of the electron accelerator are both disadvantageous.